Conventional light sources of the kind used in optical microscopy are highly inhomogeneous in themselves (for example coiled filaments or LED arrays), with the result that diffusers, usually diffusion disks, are regularly used. However, this leads to a loss of light in the direction of the object, so that the light source has to be correspondingly brighter.
In simple microscopes so-called critical illumination is often used which requires few optical components. Usually, at least the collector and field shutter can be dispensed with. The object is located essentially at the focal point of the condenser at the specimen end, which is irradiated with substantially parallel light over a large area. Any aperture shutter present is located substantially in the focal point of the condenser at the lamp end. Inhomogeneities in the far field of the light source are directly visible in the object image. If the area of the light source is too small, vignetting occurs in the object image.
However, the provision of light sources that have a sufficiently large area while at the same time being homogeneous is very costly. Particularly with higher quality microscopes with greater demands on the optical quality, such light sources can only be provided at very great expense and effort.
To be able to supply sufficient light intensity for high magnifications, strong lighting means have to be used. LEDs are popular as compact lighting means which have a number of advantages. However, it is normally necessary to use a plurality of LEDs to achieve sufficiently high intensity lighting.
In order to be able to provide sufficient homogeneity, particularly for different magnifications, diffusers, usually diffuser disks, have to be used as the interstices between the LEDs in particular lead to significant inhomogeneity. However, the use of a diffuser disk leads to a loss of light, making it necessary to use brighter LEDs and/or more of them.
In order to be able to provide sufficient illumination without vignetting, the light sources have to be enlarged. This requires on the one hand a lens system and on the other hand a relatively long optical path, which makes it necessary to fold the optical path. Both of these increase the cost enormously.
The provision of good quality critical illumination is therefore very expensive, which is why in high quality microscopes essentially only so-called Koehler illumination is used, which makes few demands of the light source. However, additional optical elements are required for this.
It is desirable to have sufficiently homogeneous critical illumination for high quality optical microscopes at low cost.